1. Field of the Invention
The invention relates to a device for monitoring an area of coverage around a working tool.
2. Related Art
The working tool can be an assembly robot, for example, which is used for assembling work pieces. Assembly robots of this type are potentially very dangerous to the operator because the operator normally must insert the work pieces manually into the assembly robot.
Protective devices are generally used to secure such areas of coverage, which devices stop the operation of the working tool in case of an unauthorized operator intervention.
These protective devices in particular can be designed as two-hand controls. Two-hand controls of this type protect the operator's hands because two levers must be activated to trigger and maintain a movement that can be dangerous. As a result, the operator must keep both hands on the levers and is thus protected from injuries during the operation of a working tool. However, the disadvantage of this arrangement is that other individuals in the area of coverage are not protected.
In cases where the working tool is designed as an assembly robot, it is a further disadvantage that the operating cycle of the assembly robot is reduced, thus resulting in an unnecessary restriction of its availability.
Safety light curtains and/or laser scanners are also used as protective devices. A safety light curtain of this type is known, for example, from reference DE 39 39 191 C3. These safety light curtains consist of several pairs of transmitters and receivers that respectively form a light barrier. As a rule, laser scanners serve as protection against stepping behind a light barrier or a light grid.
Safety devices of this type are generally used to monitor a level, two-dimensional area of coverage.
The disadvantage of these types of safety devices is that a plurality of safety devices are required to completely secure the area of coverage around an assembly robot, in particular with respect to access protection and securing of the area. Not only does this represent an undesirably high expenditure, it also requires an involved assembly, wherein the space available for installation of these devices is frequently insufficient.
Reference EP 0 902 402 A2 discloses a method for optically monitoring a three-dimensional space, in particular the door region of an elevator, with the aid of a camera. Alternatively, several cameras can be provided for monitoring respectively different segments of the three-dimensional space. The camera or each camera is designed as a digital camera and is provided with a camera module, an analog/digital converter, an evaluation unit, a micro-processor and an image storage unit. In order to monitor the three-dimensional space, a pixel-type comparison of the grey values of an actual image and the grey values of a reference image takes place in the evaluation unit. In the process, the differences are formed between the grey values of the actual image and the grey values of the reference image. In addition, the micro-processor monitors the camera operation by determining whether a basic brightness for the actual image is not reached and/or is exceeded.
Reference DE 101 20 773 A1 discloses an arrangement comprising a video camera and a device for preventing accidents, wherein the video camera is used to monitor a three-dimensional space. Alternatively, several video cameras can be provided for monitoring respectively different spatial areas. The image-processing system is coupled to or associated with the camera or to each camera. An analog/digital converter, for which the output is coupled to two image-processing channels, is installed downstream of the image-processing system. Both channels are connected to a data comparison unit.